Feat: Add cache for CityObjects

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/database/CityObjectCache.java

+package de.hft.stuttgart.citydoctor2.database;
+
+import de.hft.stuttgart.citydoctor2.datastructure.CityObject;
+import de.hft.stuttgart.citydoctor2.datastructure.GmlId;
+
+import java.util.List;
+import java.util.Map;
+import java.lang.ref.SoftReference;
+import java.util.concurrent.ConcurrentHashMap;
+
+
+/**
+ * Soft-reference cache for
+ */
+public class CityObjectCache {
+    private final Map<GmlId, SoftReference<CityObject>> cache = new ConcurrentHashMap<>();
+
+    public void put(GmlId id, CityObject cityObject) {
+        cache.put(id, new SoftReference<>(cityObject));
+    }
+
+    public void cache(List<? extends CityObject> objects) {
+
+    }
+
+    public CityObject get(GmlId id) {
+        SoftReference<CityObject> ref = cache.get(id);
+        if (ref == null) {
+            return null;
+        }
+        CityObject cityObject = ref.get();
+        if (cityObject == null) {
+            cityObject = DatabaseHandler.unmarshallCityObject(id);
+            if (cityObject == null) {
+                cache.remove(id);
+            } else {
+                this.put(id, cityObject);
+            }
+        }
+        return cityObject;
+    }
+
+    public void remove(GmlId id) {
+        cache.remove(id);
+    }
+
+    public void clear() {
+        cache.clear();
+    }
+
+    public int size() {
+        return cache.size();
+    }
+}
+